2.3. Use of --fpu option

2.3. Use of `--fpu` option

The following guidelines can be used to help you select the most suitable floating-point build options to use for your application.

ARM state only floating-point (ARMv6 and earlier)

If all of your floating-point processing is done in ARM state code and none is done in Thumb state code then:

Build your ARM code with ‑‑fpu vfpv2 (if you are using VFP9-S, VFP10 rev1 or VFP11) or with ‑‑fpu vfpv1 (to RVDS 2.1 only) if you are using VFP10 rev 0.
Build your Thumb code with ‑‑fpu none. This ensures that no floating-point operations take place in Thumb code.

ARM and Thumb floating-point (ARMv6 and earlier)

If your application will be carrying out floating-point operations in both Thumb and ARM state code, then there are a number of choices as to which options you should use.

Compiling a file using the --fpu vfpv2 switch will generate ARM code for functions that contain floating-point operations regardless of whether the file is being compiled for ARM or Thumb. This is because Thumb code cannot contain VFP instructions or access VFP registers. If the file is being compiled for Thumb any functions within the file that do not use floating-point will be compiled as Thumb code.
Compiling a file using the --fpu softvfp+vfpv2 switch means that all functions are compiled to use software floating-point linkage. This means that floating-point arguments are passed to (and returned from) functions in integer registers. Thumb code functions that contain floating-point operations are compiled as Thumb (unlike ‑‑fpu vfpv2) but will call library functions to carry out the floating-point operations in ARM code.

The choice of options that provides the best code size/performance depends upon the code being compiled. For example there is a trade-off between compiling --fpu softvfp+vfpv2 (as opposed to --fpu vfpv2) to obtain Thumb code and the corresponding inclusion of library code. It is best to experiment with different options to find the combination which provides the required code size/performance attributes.

Note

If software floating-point linkage is being used both the calling function and the called function must either be compiled using --fpu softvfp+vfpv2 or declared using the __softfp keyword.

The __softfp keyword allows software floating-point linkage to be specified on a function by function basis rather than across the whole file as is the case with --fpu softvfp+vfpv2.

See the description of __softfp in the RVDS Compilers and Libraries Guide for more information.

ARM and/or Thumb-2 floating-point (ARMv7, RVDS 3.0 and later)

VFP is directly accessible from both the ARM and Thumb-2 instruction set on ARMv7 processors with VFPv3 such as Cortex-A8. This allows you to use Thumb-2 for your entire program, without needing to switch to ARM state to perform VFP operations. To do this, build your code with thumb ‑‑fpu vfpv3. Alternatively, you can build your code for ARM only with arm ‑‑fpu vfpv3.

To build code that is compatible with other code that specifies software floating-point linkage, use ‑‑fpu softvfp+vfpv3 instead of ‑‑fpu vfpv3.

Copyright © 2005. All rights reserved.	DAI0133B
PDF version All files